If a denture the fitness of which has become worse to an oral mucosa is used, not only a person who uses it feels a pain at the time of mastication, but also the denture tends to readily come off. In such a denture, its fitness is amended by using a dental material called a denture base lining material. That is, since when an operation that a denture base lining material is built on the mucosal surface of a denture base and inserted and set in the oral cavity is carried out, the denture base lining material flows corresponding to the shape of the oral cavity and cures, a denture the fitness of which is amended good is obtained. However, since the excessive denture base lining material flows into the cervical and interdental portions remaining in the oral cavity and cures, there was a risk that the denture can not be taken off from the oral cavity.
In order to avoid such a problem, a photocuring denture base lining material has been used. This photocuring denture base lining material does not cure unless it is irradiated with visible light beams. This characteristic is extremely effective for the adjustment operation by a lining material to obtain a denture with superior fitness which hardly comes off. That is, since the shape in the oral cavity changes at the time of mastication or utterance, a denture produced corresponding to the state that a mouth is softly closed sometimes comes off at the time of mastication or utterance. In order to prevent such a problem, it is necessary to adjust the denture base lining material while having a patient masticate or utter. However, since this operation takes a long period of time. In case that a self-curing denture base lining material which polymerizes and cures within a short period of time is used, one can not undergo the adjustment to a satifactory extent and the photocuring denture base lining material is rather convenient.
The composition of the photocuring denture base lining material having such advantages does not differ at all from and is the same as the self-curing denture base lining material except for the type of a catalyst to be used. For this reason, the photocuring denture base lining material has problems common to the self-curing denture base lining material. That is, for conventionally used denture base lining materials, methyl methacrylate and isobutyl methacrylate are used as major components of the liquid, as described in Hirasawa et al., "General Remarks: The present state of commercially available various rebasing materials and as its material science arrangement", in Ouintessence of Dental Technology, Vol. 12/1987, December, p. 1475. Since these monomers have irritation against the mucosa and unpleasant odors, a problem that the adjustment of the denture base is very unpleasant to the patient still remains.
Nevertheless, the reasons why the monomers having such defects are still used are as follows. That is, since the denture base lining material must flow into small spaces between the denture base and the mucosa in the oral cavity and cure, the viscosity during building of the denture base should be low. However, since if the viscosity is low for long, the denture base lining material hangs down in the oral cavity and such is difficult for use, and the monomer is set up such that the viscosity rapidly increases. Specifically, if it is set up such that the viscosity reaches 1,500 Pa.sec 8 to 15 minutes after intimate mixing, the dental base lining material is easily used from the clinical viewpoint. Such properties are obtained by the fact that the liquid component of the denture base lining material dissolves therein methacrylic acid ester polymers as major components of the powder. Thus, methyl methacrylate and isobutyl methacrylate which have very high solubility of the methacrylic acid ester polymers and give an appropriate viscosity are used for the denture base lining material.
In recent years, in order to reduce the unpleasant odors and irritation which such materials have, denture base lining materials using neither methyl methacrylate nor isobutyl methacrylate were proposed, a part of which have been made commercially available as a low-irritation lining material. In Japanese Patent Laid-Open Nos.62-178502, 3-74311, 3-206012, 3-206013, 4-29911, 6-48912, and 6-56619, monomers having low irritation and no unpleasant odors are disclosed in place of methyl methacrylate or isobutyl methacrylate. In general, if the number of carbon atoms in the alkyl moiety of a polyalkyl methacrylate is large, the brittle temperature is greatly low as compared with that in methyl methacrylate or isobutyl methacrylate. Since the monomers having low irritation and no unpleasant odors as disclosed in the patents as cited above have a high molecular weight and have a long side chain in terms of polymers as compared with methyl methacrylate and isobutyl methacrylate, the brittle temperature tends to be low. For this reason, if these monomers are applied as the liquid component of the denture base lining material, in order to increase the solubility of the polymers and rapidly bring about an increase in viscosity after intimate mixing with the powder component of the denture base lining material, the monomers must be added in large amounts.
However, denture base lining materials using such a monomer as the major component of the liquid component are soft and are readily scratched. The American Dental Association Specifications No. 17 is concerned with denture base lining materials and defines that the denture base lining materials must have a Knoop hardness of 10 or more. However, it is very hard to meet the requirements in this specification by applying low-irritation monomers having a high molecular weight. If the amount of a monomer to be added having a high molecular weight and having a long side chain in terms of polymers is low, while the Knoop hardness of the denture base lining materials can be increased, the increase in viscosity after intimate mixing of the power component with the liquid component is slow, and hence, such is no longer used. In order to solve such a defect, a method of adding ethanol for the purpose of dissolving the polymers is proposed as disclosed in Japanese Patent Laid-Open No.6-305930. However, the denture base lining materials cured with the ethanol contained therein cause problems such as discoloration. In the light of the above, it has been very difficult to obtain photocuring denture base lining materials having low irritation, bringing about a rapid increase after intimate mixing of the powder component with the liquid component, and having a high surface hardness.
Thus, the present inventor made extensive and intensive investigations in order to obtain a novel denture base lining material which is extremely low in irritation against the mucosa, is substantially free from unpleasant odors, keeps an appropriate viscosity after intimate mixing of the powder component with the liquid component, has an operation down time enough for undergoing precise impression taking of the shape of the mucosal surface in the oral cavity, has a high Knoop hardness after curing by photocuring, and which has no fear of discoloration.